LECTURE From Measurement of Pressure and Quantity of Air in. 7.2 From Measurement of Pressure and Quantities in the

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1 LECTURE 1 THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS: 1.0 NATURAL VENTILATION.0 PRODUCTION OF NATURAL VENTILATION 3.0 DENSITY DIFFERENCE BETWEEN THE AIR OF TWO SHAFTS 4.0 DEFINITION OF NATURAL VENTILATING PRESSURE 5.0 MOTIVE COLUMN 6.0 COMPUTATION OF NVP FROM AIR DENSITY 7.0 PRACTICAL METHODS OF DETERMINING NVP 7.1 Frm Measurement f Pressure and Quantity f Air in the Fan Drift 7. Frm Measurement f Pressure and Quantities in the Fan-Drift at Tw Different Speeds f Fan 7.3 Frm Measurement f Air Pressure at Pit-Bttm With Fan Running and Fan Stpped REFERENCES Page 1 f 9

2 1.0 NATURAL VENTILATION Air will always flw frm a regin f high pressure t a regin f lw pressure. The difference in pressure may be either by natural means r by artificial means using fan. The ventilatin in a mine due t the frmer is referred t as natural ventilatin and that due t the latter is called mechanical ventilatin. It is imprtant t understand here that, in case f mechanical ventilatin, natural ventilatin als exists. In sme situatins, natural ventilatin will aid the mechanical ventilatin whereas in sme case, it may ppse mechanical ventilatin..0 PRODUCTION OF NATURAL VENTILATION Fr natural ventilatin t take place, there must be difference in density between the air f the tw shafts. As shwn in Fig. 1, there are tw shafts frm level grund surface f same diameter (D) and same depth (H). There wn t be flw f air in the level if the temperature and pressure f air in bth the shafts are the same. In this case, the weight f air clumn in bth the shafts are the same. Fig. 1 In Fig., there are tw shafts AB and CD the tps f which are at different elevatins. During winter seasn, the shaft AB will act like a dwncast shaft due t Page f 9

3 the imaginary clumn f utside air starting frm A t E. The reasn being, A t E is cld air and denser whereas A t C is ht air and lighter. This makes AB as the dwncast shaft and CD as the upcast shaft. Fig. Nw, let us take the case f summer seasn. During summer, the utside air being warmer than that f the mine, the shaft CD will act as a dwncast shaft and AB will be the upcast shaft. This type f natural ventilatin is very effective during extreme summer and winter seasns. Hwever, during ther seasns, the natural ventilatin is ineffective. In Fig. 3, a main mechanical ventilatr (Exhaust fan) is installed at the shaft tp f shaft CD. As it is evident, during winter seasn, the natural ventilatin will aid the mechanical ventilatr. The ppsing effect will be bserved during summer seasn. Page 3 f 9

4 Fig DENSITY DIFFERENCE BETWEEN THE AIR OF TWO SHAFTS The density difference between the air f the tw shafts may be created by the fllwing factrs: Presence f firedamp in ne f the shafts which makes the air lighter. Steam jet intrduced in ne f the shafts making the air lighter. Intrducing furnaces at the bttm f the upcast shaft. The heated air cming ut at the bttm f the upcast shaft gets further rarefied by the furnace and is frced upwards. The air in the mines gets heated up due t additin f heat frm rcks, men, machinery, lights, spntaneus heating etc. Cage mvement in the shaft. Flw f cld water dwn ne f the shafts, which makes the air cler and denser. Page 4 f 9

5 Additin f gases like methane and carbn dixide in the mine air changes the density f the air. While additin f methane reduces the density f return air, additin f carbn dixide increases the density f the return air and causes the ppsite effect t take place. In multilevel mine wrkings, the leakage f denser dwncast air t the upcast shaft results in an increase in the density f upcast air, thereby reducing natural ventilatin. Cl air circulatin thrugh the dwncast shaft, increases the density f dwncast air and thus increasing natural ventilatin in the mine. 4.0 DEFINITION OF NATURAL VENTILATING PRESSURE The pressure difference which is required t cause airflw in a mine by unequal densities f weights f air clumns in mine penings and withut the aid f any mechanical cntrivance is called natural ventilatin. 5.0 MOTIVE COLUMN It is pssible t express natural ventilatin as meters f air clumn r mtive clumn, which is the height f an imaginary clumn f air that prduces a pressure equal t the difference f pressure between the bttms f dwncast and upcast shafts. Mtive clumn (h) = NVP g.ρ d (m) NVP is in Pascal ρd is the density f air in the dwncast shaft (kg/m 3 ) g is acceleratin due t gravity (m/s ) 6.0 COMPUTATION OF NVP FROM AIR DENSITY The NVP in a mine is the difference in pressures f air clumns in the dwncast and upcast shafts. Page 5 f 9

6 Therefre, NVP = P d - P u P d = Pressure f air clumn in dwncast shaft P u = pressure f air clumn in upcast shaft Let the mean density f dwncast air = ρd kg/m 3 And mean density f upcast air = ρu kg/m 3 Let the depths f bth the shafts be equal and let us dente it by D in meters Then, NVP = ρ ddg ρ udg = Dg(ρ d ρ u) Pa Nw, ρd = (B d 0.378e d ) T d kg m -3 ρu = (B u 0.378e u ) T u kg m -3 Where, B d and B u = Mean barmetric readings in KPa in the dwncast and upcast shaft respectively e d and e u = Average vapur pressures f misture in KPa in the dwncast and upcast shaft respectively T d and T u = Mean temperatures f the dwncast and upcast air clumns respectively in Kelvin Let us assume that B d = B u = B Als, let us neglect the effect f misture B Therefre NVP = Dg ( T d B 87.1T u ) x 10 3 = gdb 87.1 T u T d T u T d x 10 3 Pa Page 6 f 9

7 7.0 PRACTICAL METHODS OF DETERMINING NVP 7.1 Frm Measurement f Pressure and Quantity f Air in the Fan Drift Let, Q f = Quantity f air with fan running Q n = Quantity f air with fan stpped P f = Fan drift pressure (fan pressure) P n = NVP When the fan running, the mine resistance, R = P f+p n Q f When the fan is stpped, the mine resistance, R = P n Q n Therefre, P n Q = P f+p n n Q f Slving fr P n we get, P n = P f Q n Q f Q n 7. Frm Measurement f Pressure and Quantities in the Fan-Drift at Tw Different Speeds f Fan Let the tw different fan speeds be N 1 and N. Let P f1 and Q 1 be the fan-drift pressure and fan-drift quantity at speed N 1 f the fan. Let P f and Q be the fan-drift pressure and fan-drift quantity at speed N f the fan. Then Mine resistance, R = P n+p f1 Q = P n+p f 1 Q Page 7 f 9

8 P n = P f1 P f Q 1 Q Q 1 - P f1 Or P n = P f1 P f Q 1 Q Q - P f 7.3 Frm Measurement f Air Pressure at Pit-Bttm with Fan Running and Fan Stpped Let, p n = Pit bttm pressure with fan stpped p f = Pit bttm pressure with fan running R = Mine resistance R s = Shaft resistance Therefre, p n = P n - R sq n Nw, P n = RQ n Q n = P n R Therefre, p n = P n - R s P n R = Pn (1 - R s R ) (1) In a similar way, pf can be cmputed which will give p f = (P n + P f) (1 - R s R ) () Frm Equatins (1) and () abve, we get, p f p n = P n+ P f P n Pn = Pf p n p f p n Page 8 f 9

9 REFERENCES Banerjee S.P. (003); Mine Ventilatin ; Lvely Prakashan, Dhanbad, India. Deshmukh, D. J. (008); Elements f Mining Technlgy, Vl. II ; Denett & C., Nagpur, India. Hartman, H. L., Mutmansky, J. M. & Wang, Y. J. (198); Mine Ventilatin and Air Cnditining ; Jhn Wiley & Sns, New Yrk. Le Rux, W. L. (197); Mine Ventilatin Ntes fr Beginners ; The Mine Ventilatin Sciety f Suth Africa. McPhersn, M. J. (1993); Subsurface Ventilatin and Envirnmental Engineering ; Chapman & Hall, Lndn. Misra G.B. Calcutta, India. (1986); Mine Envirnment and Ventilatin ; Oxfrd University Press, Vutukuri, V. S. & Lama, R. D. (1986); Envirnmental Engineering in Mines ; Cambridge University Press, Cambridge. Page 9 f 9